1. Field of the Invention
The present invention relates to conveyor drive systems, in particular, to a conveyor drive system with lowered vibration, noise, or pulsation, and a motor built-in reducer used for the conveyor drive system.
2. Description of the Related Art
FIG. 9A is a top plan view and FIG. 9B is a front view of a prior art conveyor drive system, in which the reference numeral 100 denotes a conveyor and the reference numeral 200 denotes a drive system therefor.
The conveyor 100 includes a bed 101, a pair of drums 102 rotatably supported at both ends of the bed 101, and an endless conveyor belt 103 trained over both drums 102. The drawings illustrate only one end of the bed 101 and one of the drums 102. A driven sprocket 104 is coupled to the shaft of the illustrated drum 102, so that the drum 102 is driven by a geared motor 201 via a chain 105 trained over the driven sprocket 104. The reference numerals 106, 107, and 108 in the drawings represent an object being transported, rollers, and a support leg, respectively.
The drive system 200 includes the geared motor 201 having an output shaft 222 to which a drive sprocket 203 is mounted. The endless chain 105 is trained over this drive sprocket 203 and the driven sprocket 104 of the conveyor 100. The reference numeral 204 represents a base. The support legs 108 of the conveyor 100 are joined to this base 204.
The geared motor 201 consists of a motor 210 and a reducer 220. The reducer 220 may be, for example, a planetary gear reducer disclosed in Japanese Patent Laid-Open Publication No. Hei. 8-4844, a parallel shaft gear reducer using helical gears, or an orthogonal gear reducer using hypoid gears proposed by the applicants of the present invention disclosed in Japanese Patent No. 2628983. In the applications where a low noise, low vibration environment is particularly desirable, it is the practice to use a reducer employing a helical gear set or an orthogonal gear reducer employing a hypoid gear set.
For example, in the application as an inspection conveyor for the visual checking for any small foreign matter in a food product package, it is highly desirable to suppress vibration or pulsation of the conveyor belt as much as possible.
While the demand for such conveyor has been growing, no improvements have been made so far that address this problem.
The present invention has been devised under these circumstances, taking account of the problems caused by vibration, noise, or pulsation in conveyors. An object of the invention is to provide a conveyor drive system and a motor built-in reducer used for the system, by which problems resulting from vibration, noise, or pulsation in conveyors are eliminated.
To solve the above problems, the present invention provides a conveyor drive system including a motor, a reducer mechanically interconnected to the motor for transmitting rotation of the motor at a reduced rate to a drum for driving the conveyor, and a transmission unit for discretely transmitting output from the reducer to the drum. The transmission unit includes a driving rotary member, a driven rotary member, and an endless power transmission member trained over the driving rotary member and the driven rotary member, for discretely transmitting the output. The reducer is a simple planetary roller reducer having a sun roller, a plurality of planetary rollers in rolling contact with an outer periphery of the sun roller, and a ring roller, the planetary rollers being in contact with an inner periphery thereof. An output shaft of the reducer is connected to the driving rotary member of the transmission unit.
In known conveyors, generally, when one of the driving and driven sprockets (the driving rotary member and the driven rotary member) in the endless power transmission mechanism has too small a pitch circle diameter, the links of the chain trained over the sprockets (transmission unit) do not form a smooth arc but numerous sides of a polygon. As a result, the chain moves around the driving and driven sprockets as if discretely, making a rattling noise. This movement of the chain will be hereinafter referred to as a xe2x80x9cpolygonal motionxe2x80x9d throughout this specification.
Such polygonal motion is most likely to occur in power transmission using a metal chain and sprockets, but it also can occur in a belt drive using a rubber or plastic timing belt. That is, for either a chain drive or a timing belt drive, as long as power is discretely (digitally) transmitted, the polygonal motion occurs in these types of power transmission. This phenomenon has not been considered a problem so far.
Recent use however of an inverter power supply with converting frequency which enables a variable speed drive of a motor has brought about a problem of unexpected noise from not only the chain but also from the geared motor and its neighboring constituents such as a bed on which the geared motor is placed, or supporting legs and bases. This noise is apparently caused by resonance between the chain or timing belt and the driving geared motor, which occurs at certain rpm of the motor.
The inventors have ascertained through research that this problem is likely to occur particularly when the chain sprocket has a small pitch circle diameter.
This finding indicates that the resonance occurs in association with the above-described xe2x80x9cpolygonal motion.xe2x80x9d
It is generally easy to take measures to suppress noise and vibration if the level of the noise and vibration is constant. It is however, not easy to take appropriate measures to prevent noise or vibration which is generated unexpectedly depending on the situation. This is because it is not desirable in terms of cost to provide measures taken to prevent such unexpected noise or vibration, which result in excessive quality during the majority of the operation period.
In the case where the resonance is presumably caused by the polygonal motion, the problem could be solved to some extent by increasing the diameter of the sprockets. However, it is possible that objects being transported on the conveyor belt are positioned beyond the conveyor belt edge. Therefore if the sprocket is to have a larger diameter, then the drum itself or the entire conveyor must be made large in order that the sprocket does not stick out from a side of the belt. These measures however increase the equipment costs and require more space to install the system. Another option to prevent the xe2x80x9cpolygonal motionxe2x80x9d is to employ a flat smooth belt instead of a chain or a timing belt to perform analogue or seamless power transmission using friction.
However, the structure of the analogue power transmission does not allow a large torque to be transmitted, and accordingly the overall mechanism tends to be bulky. Also, the problem of slippage at the belt-to-pulley interface which is intrinsic to this mechanism makes it hard to perform feed-forward or feed-back control, and therefore precise positioning or control of speed is impossible.
The present invention presupposes the use of chains or timing belts and aims at solving the problems described above. The primary feature of the invention is that it employs a reducer of a simple planetary roller mechanism which performs traction transmission for the reducer interposed between the motor, which is the source of vibration and thus the source of resonance, and the chain or timing belt which cannot help but generate a polygonal motion. Data proving the effects of using traction transmission will be given later.